This invention relates generally to television transmission and reception systems and, more particularly, to television transmission and reception systems for broadcasts having stereo audio.
Television broadcasts in stereo audio are becoming increasingly popular. Conventional television signals, including an NTSC (National Television System Committee) television signal, typically provide only a single channel for the audio signal. For example, the NTSC audio signal is transmitted on a subcarrier frequency at 4.5 MHz above the video carrier frequency. The 4.5 MHz frequency was selected to minimize interference with the video signal, which has an upper bandwidth of approximately 4.2 MHz above the video carrier frequency. Unfortunately, the audio subcarrier frequency at 4.5 MHz is not sufficiently separated from the video upper bandwidth to easily accommodate the bandwidth required for stereo audio.
Various techniques have been devised for transmitting stereo audio within the bandwidth limitations of the conventional NTSC television signal. These techniques vary depending upon the mode of transmission of the television signal. For ground-based television transmission systems, such as over-the-air or cable transmission systems, the bandwidth of the television signal is generally limited to 4.5 MHz. One technique for transmitting stereo audio within this 4.5 MHz bandwidth limitation is to transmit the stereo audio signal during the video horizontal blanking interval. However, transmitting the stereo audio signal during the video horizontal blanking interval destroys both the video horizontal synchronization signal and the video color burst signal. In addition, the stereo audio signal is frequently digitized to increase the quality of the sound and to allow for encryption of the audio signal, thus preventing unauthorized access to the audio signal. However, the time duration of the horizontal blanking interval is insufficient to transmit a digitized stereo audio signal, unless the digitized audio signal is either compressed or the sampling rate of the audio signal is reduced below the Nyquist sampling rate. Both of these alternatives severely degrade the quality of the stereo sound.
For satellite-based transmission systems, the stereo audio signal is frequently transmitted on two separate subcarrier frequencies, at 5.8 and 7.6 MHz above the video carrier frequency. The left channel of the stereo audio signal is transmitted over one frequency and the right channel of the stereo audio signal is transmitted over the other frequency. The two channels are necessary because the conventional NTSC audio channel at 4.5 MHz interferes with the video signal and, therefore, is generally not transmitted. However, the two satellite channels are typically analog channels and cannot accommodate the bandwidth that is required to transmit a digitized stereo audio signal, without first compressing or reducing the sampling rate of the stereo audio signal.
Accordingly, there has been a long existing need in the broadcasting industry for a method and apparatus for transmitting and receiving high-fidelity digital stereo audio within the bandwidth limitations of conventional television transmission and reception systems. The present invention clearly fulfills this need.